It has been demonstrated that the in vitro culturing of normal murine spleen cells results in the generation of active suppressor cell activity. This activity is assayed as the ability to suppress the response of otherwise competent cells to alloantigens in cell-mediated lympholysis. A T cell subpopulation has been shown to be essential to this suppressor activity. Furthermore, the suppression has been shown to effect responses to transplantation antigens including alloantigens, "modified self" determinants, and syngeneic tumors.